Optical wavelength dispersive devices (WDD) often use diffraction gratings to spatially disperse different wavelength of incoming light and direct them along differing optical paths. Such grating-based WDD typically also includes an input aperture to launch light into the device, and collimating or focusing optical elements having optical power such as lenses and curved mirrors to form spectrally dispersed images on a focal surface.
One common embodiment of a WDD is a spectrograph, wherein a plurality of detector elements are disposed at the focal surface for measuring the intensity of the spectral components of the incoming light beam. Another important embodiment of a WDD is a wavelength selective switch (WSS), wherein an optical fiber is disposed at the input aperture, a plurality of controllable switching elements such as micro-electro-mechanical (MEMS) micro-mirrors are disposed at the focal surface, and the switching elements are effective in redirecting spectral components of the input optical signal back into a selected output optical fiber. Examples of such devices are disclosed in U.S. Pat. No. 6,097,859 issued Aug. 1, 2000 to Solgaard et al; U.S. Pat. No. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al; U.S. Pat. No. 6,707,959 issued Mar. 16, 2004 to Ducellier et al; U.S. Pat. No. 6,810,169 issued Oct. 26, 2004 to Bouevitch, and U.S. Pat. Publication No. 2007/0242953 published Oct. 18, 2007 to Keyworth et al, which are incorporated herein by reference.
Another example of a WDD is a wavelength blocker (WB), or a dynamic gain equalizer (DGE). In these devices, dispersed images corresponding to de-multiplexed wavelength channels may be formed upon an array of liquid crystal cells, which independently rotate the state of polarization of the wavelength channels to either partially attenuate or completely block selected channels from passing back through the polarization diversity unit in the front end. Examples of WB and DGE backend units are disclosed in U.S. Pat. No. 7,014,326 issued Mar. 21, 2006 to Danagher et al; U.S. Pat. No. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al; and U.S. Pat. No. 6,810,169 issued Oct. 26, 2004 to Bouevitch, which are incorporated herein by reference.
Another example of a WDD is a fiber optic multiplexer/demultiplexer, where an optical fiber is disposed at the input aperture, and a plurality of output optical fibers are disposed at the dispersed focal plane to receive the dispersed spectral components.
FIG. 1 illustrates a top view of a typical platform 100 for a WDD in which a spherical reflector 120 receives a beam of light from a front-end unit 122. The spherical reflector 120 reflects the beam of light to a diffraction grating 124, which disperses the beam of light into its constituent wavelength channels. The wavelength channels are again redirected by the spherical mirror 120 to a backend unit 126.
In the case of a WB or a DGE the front end unit 122 can include a single input/output port with a circulator, which separates incoming from outgoing signals, or one input port with one output port. Typically the front end unit 122 will include a polarization diversity unit for ensuring the beam (or sub-beams) of light has a single state of polarization. The backend unit 126 for a WB or a DGE includes an array of liquid crystal cells, which independently rotate the state of polarization of the wavelength channels to either partially attenuate or completely block selected channels from passing back through the polarization diversity unit in the front end 122.
In the case of a wavelength selective switch (WSS) the front end unit 122 is illustrated in FIG. 2 and includes an array 132 of input/output fibers 132A to 132D, each of which may have a corresponding lens 134A to 134D, respectively, forming a lens array 134. An angle to offset, or switching, lens 136 converts the lateral offset of the input fibers 132A to 132D into an angular offset at a point 138, which is imaged by the spherical lens 120 onto the backend unit 126. The lens array 134 can be removed depending on the relative positions of the switching lens 136. The backend unit 126 in a WSS is typically a MEMS array of tilting mirrors which can be used to steer each of the demultiplexed beams to one of several positions corresponding to a desired output port. The angle introduced at the back end unit 126 is then transformed by the angle to offset lens 136 to a lateral offset corresponding to the desired input/output fiber 132A to 132D. Alternatively, a liquid crystal phased array (LC or LCoS, if incorporated on a silicon driver substrate) can be used to redirect the light.
In operation as a WSS, a multiplexed beam of light is launched into the front-end unit 122 and optionally passes through the polarization diversity unit formed of a polarization beam splitter and a waveplate to provide two beams of light having the same state of polarization. The two beams of light are transmitted to the spherical reflector 120 and are reflected therefrom towards the diffraction grating 124. The diffraction grating 124 separates each of the two beams into a plurality of channel sub-beams of light having different central wavelengths. The plurality of channel sub-beams are transmitted to the spherical reflector 120, which redirects them to the MEMS or LC phased array 126, where they are incident thereon as spatially separated spots corresponding to individual spectral channels.
Each channel sub-beam can be reflected backwards along the same path or a different path, which extends into or out of the page in FIG. 1 to the array of fibers 132, which would extend into the page. Alternatively, each channel sub-beam can be reflected backwards along the same path or a different path, which extends in the plane of the page of FIG. 1. The sub-beams of light are transmitted, from the MEMS or LC phased array 126, back to the spherical reflector 120 and are redirected to the diffraction grating 124, where they are recombined and transmitted back to the spherical reflector 120 to be transmitted to a predetermined input/output port shown in FIG. 2.
In all such devices it is often desirable to have the positions of the spectrally dispersed images on the focal surface remain fixed as the temperature of the device is varied. However, as the temperature of the optical system of the WDD changes, the optical beam path through the WDD may vary, resulting in shifts of the dispersed images at the focal surface. In principle, it should be possible to choose optical materials and design mechanical support structures to make the positions of the spectrally dispersed images on the focal surface invariant over temperature. In practice however, this is often difficult or impossible, as materials with required thermal characteristics may not exist, or may have other properties or costs which make them impractical. Furthermore, modifying the design of the imaging optics to achieve temperature invariance may compromise the imaging properties of the optical system.
It is therefore an object of the present invention to provide means that would compensate for the temperature dependence of the optical system of a WDD without significantly complicating its optical design or significantly increasing its cost.
It is another object of the present invention to provide a WDD having a dispersive subsystem which produces a pre-determined non-zero shift in diffracted angle over temperature for passive compensation of temperature-induced variations in the device performance.